1. Field of the Invention
The present invention relates to a semiconductor laser device, in particular, to a high output semiconductor laser device whose oscillation wavelength is 600 nm or less.
2. Description of the Related Art
Presently, high output semiconductor laser devices are in demand for use in writing onto optical discs and magnetic optical discs. Such semiconductor laser devices are required to operate stably for long time in a fundamental mode.
Also, in order to realize the shorter wavelengths necessary for higher recording density of optical discs, semiconductor laser devices using nitride semiconductors have been researched. The laser devices using the nitride semiconductors can be applied not only as a light source for optical discs, but also as light sources for exposure, printing, medical use, optical communication systems, measurement systems and the like. Also, with the laser devices having nitride semiconductors, the use in the ultraviolet region with the oscillation wavelength of 400 nm or less becomes possible, so that the application as a light source or the like in the bio-related area is also expected.
Japanese Unexamined Patent Application Publication No. H09-162497 discloses a configuration in which an Al2O3 layer of λ/2 in thickness is disposed on the reflecting mirror of the laser device, and a configuration in which an Al2O3 layer of λ/4 in thickness and an amorphous silicon layer of λ/4 in thickness are alternately stacked on the reflecting mirror of the laser device. Japanese Unexamined Patent Application Publication No. 2002-335053 discloses the use of Al2O3 as a dielectric film and the a method for disposing thereof.
However, when the Al2O3 layer of λ/2 in thickness is formed with a single layer, an amorphous Al2O3 layer may react with the semiconductor device under high output drive of 300 mW or higher, which leads to degradation of the end surfaces. Also, because of a large stress in a single crystal material Al2O3 film, heat generated during driving leads to separation of the Al2O3 film from the semiconductor device.
Also, in the semiconductor laser devices using the nitride semiconductors, if the dielectric layer of the end surface of the light emitting side is made of a single layer, destruction of the end surface tends to occur under high output operation such as at 30 mW or higher, resulting in a shortening the lifetime of the device. Moreover, in high output driving, the driving current becomes too large when the slope efficiency is low. A semiconductor laser device begins to oscillate the laser when strength of current applied to the laser device is increased and reaches a certain strength of current. As the current is increased, the laser output power is also increased. The slope efficiency usually means the ratio (ΔP/ΔI) of the increase of the laser output power (ΔP) to the increase of current (ΔI). The semiconductor laser device having the higher slope efficiency can obtain higher laser output power with lower current. On the contrary, a semiconductor laser device having lower slope efficiency needs higher current to obtain the higher laser output power. When the greater current is applied, heat can be generated at the laser device, and the heat can degrade or destroy the crystal. A reflectance ratio of the end surface of the light emitting side (front) and the slope efficiency can be adjusted by adjusting the refractive index and thickness of dielectric layer.